1. Prior Art
To increase the efficiency of filter media, for example filter units in stationary or mobile dust removal systems or also in ventilation and air conditioning systems, numerous efforts have been made in the past.
In DE 19919809 A1, a dust filter bag for use in vacuum cleaners is described, which consists of at least one nonwoven layer and at least one support material layer and which is suitable for the efficient removal of fine dust particles without significantly reducing the cleaning power of the vacuum cleaner. However, due to the relatively thin filter paper layer, nanofiber layer, and meltblown layer that are used, the collection capacity of the material is low as dust particles can collect only in the surface region of the filter.
A multi-layer vacuum cleaner filter bag with a very good dust removal action is also revealed in EP 0960645 A2. The layer structures with their varied designs generally contain one wet-laid or dry-laid filter paper layer, or one voluminous meltblown nonwoven, or one spunblown nonwoven, or one fine-denier spun-laid nonwoven.
A shortcoming is its complex structure of different materials, which necessitates greater expenditures during the production.
EP 0822775 B1 presents a shock resistant vacuum cleaner filter bag containing a filter laminate that is composed of an outer support layer, a fibrous nonwoven filter layer, and an inner diffusion layer. The nonwoven filter layer is preferably a meltblown electret microfiber nonwoven web, whereas the support and diffusion layer may be a spun-laid nonwoven or a hardened carded web of fabric.
They all have in common the idea of utilizing, in addition to a stabilizing layer, a plurality of filter layers that have filtering capacities of varying degrees of filtration efficiency, i.e., each layer filters out particles of a certain size from the dust mixture. The shortcoming is that two different fractions of dust particles are located directly next to one another at each boundary surface between the individual filter layers since the individual filter layers are filled with dust from the inflow to the outflow side.
2. Object and Summary of the Invention
FIG. 1 shows the distribution of the dust particles within such a multi-layer filter according to the prior art. An explanation of the reference letters can be found in the list of reference letters.
This distribution of the dust collection that is characteristic for filter media produced according to the prior art, of the type used for vacuum cleaner bags according to the preamble causes a significant increase in the differential pressure, particularly at the beginning and towards the end of their time in use. This results in a reduced cleaning power of the vacuum cleaner and, hence, also in a reduced dust collection efficiency.
The invention, therefore, has as its object to provide a multi-layer filter material that avoids the above shortcomings of the prior art. This object is met in such a way that a multi-layer filter structure (E) is used for the dust extraction from gases, wherein a coarse dust filter layer (A), a fine dust filter layer (B) and a support layer (C) are disposed one after another in the direction from the inflow side to the outflow side, which are designed such that the fiber diameter distribution within the coarse dust filter layer (A) and fine dust filter layer (B) has a gradient and the fiber diameters of the coarse dust filter layer (A) and fine dust filter layer (B) continually decrease from the inflow side to the outflow side.
To attain the highest possible air flow rate as the dust load increases, it is necessary that the dust is deposited as homogeneously as possible across the entire thickness of the filter structure and that no boundary surfaces are contained in the filter structure at which an accumulation of dust particles results in a blockage of the entire structure.
This is attained with a continuous decrease in pore size from the inflow side to the outflow side across all layers forming the filter material, without the presence of clearly noticeable transitional areas.
The pores in a nonwoven exist as hollow spaces between the fibers, i.e., the smaller the selected fiber diameter and/or the higher the selected compaction of the nonwoven, the smaller the resulting pore size. To prevent an abrupt transition due to different pore sizes in the region of the contacting surfaces of the filter layers it is necessary that the contacting filter regions have approximately the same fiber diameter.
The fractional distribution of the deposited dust particles across the entire cross section of the filter medium is made possible in such a way that the filter medium has increasingly smaller pores in each filter layer from the inflow side to the outflow side, so that large dust particles are absorbed by corresponding large pores and small dust particles by small pores.
This desired progressive filter design is attained by forming a fiber diameter gradient, as well as by selecting special forming and hardening methods for the nonwovens and combining suitable nonwovens.